The present invention relates to a carbonylation process improvement. More particularly, this invention relates to an improved process scheme wherein alkanes are removed from carbonylation products.
Recently, processes for producing carboxylic acids and esters by carbonylating olefins, alcohols, esters, halides and ethers in the presence of homogeneous catalyst systems that contain rhodium or iridiium and halogen components such as iodine components and bromine components have been disclosed and placed into commercial operation. These recently developed processes represent a distinct improvement over the classic carbonylation processes wherein such feed materials have been previously carbonylated in the presence of such catalyst systems as phosphoric acid, phosphates, activated carbon, heavy metal salts and metal carbonyls such as cobalt carbonyl, iron carbonyl and nickel carbonyl. All of these previously known processes require the use of extremely high partial pressures of carbon monoxide. These previously known carbonylation systems also have distinct disadvantages in that they require higher catalyst concentrations, longer reaction times, higher temperatures to obtain substantial reaction and conversion rates that all result in larger and more costly processing equipment and higher manufacturing costs.
The discovery that rhodium or iridium and iodine- or bromine-containing catalyst systems will carbonylate such feed materials as olefins, alcohols and ester, halide or ester derivatives of the alcohols at relatively mild pressure and temperature conditions was a distinct contribution to the carbonylation art. In spite of the vast superiority of these newly developed catalyst systems, it was found that conventional processing schemes for separation of the carbonylation products from the liquid reaction mass posed problems of catalyst inactivation and precipitation. In U.S. Pat. No. 3,845,121, an improved process scheme is described wherein carbonylation products can be recovered from a carbonylation reaction zone without resulting in decomposition of the catalyst system when the liquid-phase active catalyst and unreacted feed components are recycled to the reaction zone. According to this process, the carbonylation rection is carried out in the reaction zone at a temperature from about 50.degree. to 150.degree. C. and a pressure from about 50 to 1500 psia and at least a portion of the liquid reaction mass is passed to a separation zone without the addition of heat, said separation zone having a pressure of at least 20 psi less than the pressure in said reaction zone to vaporize at least a portion of the carbonylation products, the vaporized carbonylation products being withdrawn and the remaining liquid reaction mass being recycled to said reaction zone.
The vaporized product obtained in the process of U.S. Pat. No. 3,845,121 is passed to a conventional purification system of multiple distillation columns for ultimate recovery of the carboxylic acid in pure form. However, certain difficulties arise during such purification which are attributable to the presence in the system of alkanes typical among which are those having from 6 to 12 carbon atoms and which are straight-chain compounds such as hexane, heptane, octane, decane and the like. Alkanes can enter the carboxylic acid production facilities from several sources among which are the carbon monoxide and the methanol feed streams. The alkanes azeotrope or steam distill with water and are soluble in methyl iodide. They create problems in the reaction zone in that a second phase containing alkanes, methyl iodide and acetic acid is generated therein. In addition, the alkanes appear in the heavy phase from the overhead of the column used to remove the light ends from the reaction effluent. This heavy phase is normally recycled to the reactor. In addition, to the fact that the presence of alkanes therein has a tendency to overload the heavy phase pump due to dramatic density changes, these unwanted compounds tend to load down the light ends column itself thus limiting plant capacity. There is no way out of the system for these alkanes except by actual spills and thus a need for a method for their removal is created.
It is an object of the present invention, therefore, to provide a method for the removal of alkanes in a process wherein carboxylic acids are produced by the reaction of an alcohol or olefin with carbon monoxide in the presence of a rhodium- or iridium-containing catalyst system and recovered therefrom by distillation.
Other objects and advantages of the invention will become apparent from the following discussion of the invention.